1. Field of the Invention
The present invention relates to an internal-combustion engine having an electronically-controlled throttle valve, and more particularly to a throttle valve control device in an internal-combustion engine for electronically controlling the throttle valve so that the throttle valve opening is adjusted to a final target opening, which is calculated based on the engine operation state and an ISC request opening.
2. Description of the Related Art
During an idling state of an engine, a so-called ISC (Idling Speed Control) is performed. In ISC, an ISC valve is controlled so as to adjust the revolutional speed of the engine to the target revolutional speed. In ISC using an ISC valve, the idling revolutional speed is maintained at a target value by controlling the air amount (or intake amount) supplied to the engine independently of the throttle valve. In order to enhance the responsiveness of ISC, it has been proposed to correct an ISC amount based not only on the revolutional speed of the engine but also based on the variation in the revolutional speed (see, for example, Japanese Laid-Open Patent Application No. 7-34940).
In recent years, as the electronic control of the throttle valve of an internal-combustion engine has been realized, it has been proposed to perform ISC using the electronically-controlled throttle valve without separately providing an ISC valve. When such ISC is performed using a throttle valve, the idling opening of the throttle valve (i.e., the opening of the throttle valve during an idling state of the engine; hereinafter, referred to simply as the "idling opening") is set based on the variation in the throttle valve opening caused by ISC.
In such a throttle valve control, it is determined if the engine is in an idling operation based on predetermined conditions; and if so, ISC is performed as described above. The idling state determination is based on a comparison using a predetermined threshold value, a mechanically-designed idling switch of the throttle valve, or the like.
When a driver slightly depresses the accelerator during the idling operation of an internal-combustion engine, in response to this, the revolutional speed of the engine increases. However, if the throttle valve opening at that time does not exceed the idling opening range (i.e., the predetermined threshold value), it is still determined that the engine is in an idling operation, whereby the ISC amount (i.e., the throttle valve opening) is corrected in accordance with the variation in the revolutional speed which is caused by the depression of the accelerator. More specifically, the ISC amount is corrected so as to compensate for the increase in the revolutional speed caused by the depression of the accelerator. If the driver then removes their foot from the accelerator, the throttle valve opening required for an idling operation is no longer obtained, thereby lowering the revolutional speed of the internal-combustion engine.
Since the conventional throttle valve control device cannot reliably detect slight variances in the opening/closing of the accelerator, as described above, a slight depression of the accelerator during the idling operation lowers the stability of the revolutional speed of an internal-combustion engine.